The present disclosure relates generally to imaging devices, and more particularly to a system and method for image-based attenuation correction of PET and/or SPECT images.
Hospitals and other health care providers rely extensively on imaging devices such as computed tomography (CT) scanners, magnetic resonance imaging (MRI) scanners, single photon emission computed tomography (SPECT) scanners, and positron emission tomography (PET) scanners for diagnostic purposes. These imaging devices provide high quality images of various bodily structures and functions. Each imaging device produces a different type of image based on the physics of the imaging process. For example, in a CT scanner, an x-ray source generates x-rays which propagate through the body and are detected by a detector on the other side of the body. The x-rays are attenuated to different degrees depending on what bodily structures they encounter, which results in an image showing the structural features of the body. CT scanners, however, are not particularly sensitive to biological processes and functions.
PET scanners, on the other hand, produce images which illustrate various biological processes and functions. A typical emission scan using a PET scanner starts with the injection of a solution including a tracer into the subject to be scanned. The subject may be human or animal. The tracer is a pharmaceutical compound including a radioisotope with a relatively short half-life, such as 18F-fluoro-2-deoxyglucose (FDG), which is a type of sugar that includes radioactive fluorine. The tracer has been adapted such that it is attracted to sites within the subject where specific biological or biochemical processes occur. The tracer moves to and is typically taken up in one or more organs of the subject in which these biological and biochemical processes occur. For example, when the tracer is injected, it may be metabolized by cancer cells, allowing the PET scanner to create an image illuminating the cancerous region. When the radioisotope decays, it emits a positron, which travels a short distance before annihilating with an electron. The short distance, also called the positron range, is typically of the order of 1 mm in common subjects. The annihilation produces two high energy photons propagating in substantially opposite directions. The PET scanner includes a photon detector array arranged around a scanning area, usually in a ring-shaped pattern, in which the subject or at least the part of interest of the subject is arranged. When the detector array detects two photons within a short timing window, a so-called ‘coincidence’ is recorded. The line connecting the two detectors that received the photons is called the line of response (LOR). The reconstruction of the image is based on the premise that the decayed radioisotope is located somewhere on the LOR. It should be appreciated that the annihilation occurs on the LOR and the decayed radioisotope is a positron range removed from the point of annihilation. The relatively short positron range may be neglected or may be compensated for in the reconstruction. Each coincidence may be recorded in a list by three entries: two entries representing the two detectors and one entry representing the time of detection. The coincidences in the list may be grouped in one or more sinograms. A sinogram is typically processed using image reconstruction algorithms to obtain volumetric medical images of the subject. Despite such benefits, PET scanners, however, do not generally provide structural details of the patient as well as other types of scanners such as CT and MRI scanners.
Recently PET-CT scanners have been introduced. A PET-CT scanner includes both a CT scanner and a PET scanner installed around a single patient bore. A PET-CT scanner creates a fused image which comprises a PET image spatially registered to a CT image. PET-CT scanners provide the advantage that the functional and biological features shown by the PET scan may be precisely located with respect to the structure illuminated by the CT scan. In a typical PET-CT scan, the patient first undergoes a CT scan, and then the patient undergoes a PET scan before exiting the scanner. After the CT and PET data have been acquired, the PET-CT scanner processes the data and generates a fused PET-CT image.
In normal practice, PET or SPECT images are reconstructed using attenuation correction. This is essential for quantification. For example, attenuation correction takes into account the fact that photons may be scattered by body parts so that these photons are not detected. Scattered photons that are detected may also need to be taken into account. This process is generally called “scatter correction.”
Attenuation correction requires an estimate of the properties of the attenuation medium (e.g., density). This is typically based on an additional measurement, e.g. transmission scan or CT, or some other calculation or data. If the estimate is inaccurate, the resulting emission images will show artifacts. A common problem, for instance, is patient movement between the PET and CT scan (e.g., global movement, cardiac motion (heartbeat), respiratory motion (breathing), etc.). This may result in problems in data analysis. For example, cardiac scans may show a defect in the myocardium, which is only due to misalignment of the heart between PET and CT. Another example of misalignment error includes lung tumor quantification.
Currently, PET data may be gated for respiratory motion, obtaining different data sets for different stages in a breathing cycle (e.g., mid-inspiration, end of inspiration, mid-expiration, end of expiration, etc.) such that respiratory motion no longer influences the images. A major problem is obtaining matching attenuation correction. Some potential solutions may include: deformation of a fast CT (performed at breathhold) to match the PET images; CINE CT with an afterwards averaging of CT slices acquired at different time-points (and hence breathing stages); CINE CT processed to obtain CT images in corresponding stages of the breathing cycle. In many cases, however, a remaining mismatch between the CT and PET may still be observed.
Current techniques to correct for errors in the attenuation estimate require re-reconstruction of the emission images. For example, a scanner console may include a semi-automatic method, incorporated into a software program (e.g., on GE™ PET/CT scanners this is part of a CardIQ™ software package) to realign cardiac CTs to the PET image after which a second reconstruction is performed.
Re-reconstruction is not always practical or possible as it requires access to the raw emission data, and fast processing hardware. Accordingly, existing systems do not guarantee reliable results in situations where such raw emission data are not readily available.
The present disclosure provides a system and method for addressing these deficiencies. As a result, techniques for efficiently and practically correcting misalignment and reconstructing accurate and reliable images in PET and/or SPECT using images rather than raw emission data is provided in the present disclosure. In one embodiment, the proposed system and method may be implemented offline. For example, a clinician reviewing the data may notice a mismatch between the attenuation and emission image. The method and system may allow for correction of this mismatch at a workstation, without access to the raw data. This makes the correction an image-based correction, which may be much easier to incorporate into the clinical workflow than existing methods. In addition, because CT to PET registration may be problematic resulting from errors in an emission image due to attenuation mismatch, it may also be advantageous for these errors to be immediately corrected to create an updated image. These updated images may be used for further improvement of registration. Thus, embodiments of the present disclosure may provide an improved registration technique for providing image-based correction of PET and/or SPECT images, after which a final reconstruction of the PET and/or SPECT images may be performed with the corrected attenuation image.